Production of titanium compounds and metal by sustainable Methods

ABSTRACT

A unique production of titanium compounds and metal by sustainable methods using iron-titanium oxide starting material such as ilmenite, leucoxene, or rutile is described. Here the iron-titanium oxide compound is prepared by converting the iron portion of the compound to ferrous chloride at low temperatures by using close to stoichiometric amounts of sulfur and chlorine required for all the iron oxides and the other non-titanium oxides. The ferrous chloride thus formed is removed recovering a marketable product of ferrous chloride and the ‘sustainable’ titanium oxide starting material by additional process steps. This can be converted to ‘sustainable’ titanium metal, or titanium tetra-chloride by process shown herein for further conversions to titanium dioxide pigment by present chloride process or supplied to existing titanium sponge producers, benefitting them in having a ‘sustainable process’.

BACKGROUND

During the past seventy years titanium metal has been carried out usingKroll reduction of titanium tetra-chloride [TiCl₄] using magnesium [Mg]metal at about 900° C. Large quantity production of each batch of about8 tons of titanium sponge takes a cycle time of about four days. Thetitanium tetra-chloride production is done from naturally occurring hightitanium oxide rutile [˜92-94% TiO₂], or high TiO₂ containing slagcalled synthetic rutile[82-88% TiO₂] processed from ilmenite [FeTiO₃] orother iron-titanium oxide raw material containing between 45 to 60%TiO₂. Conversion of iron-titanium oxide has been typically carried outby a carbo-thermic process emitting CO₂, and consumes energy which ismostly from fossil fuel source . Prior to carrying out the magnesiumreduction, the titanium oxide is chlorinated using coke and chlorine at800 to 1400° C. depending on the process. Additional steps purifyundesirable impurities from TiCl₄. This step also emits CO₂. Since thestep of magnesium reduction of TiCl₄ is exothermic—and the management oftemperature control is done with water cooling during portions ofreaction, the external energy consumption in this step is minimal;however, an average of 900° C. is considered the reduction temperature.The products of reaction are magnesium chloride melt and spongetitanium. The magnesium chloride melt is typically, recycled to producethe metallic magnesium needed for further reduction in most cases whenthe price of magnesium is high; otherwise solidified magnesium chlorideis sold as a separate product (as practiced in China).

During the past 20 years, several processes based on calcium as areductant utilizing electrolysis as a method have been developed andnone has come to a production scale of even 100 kg of titanium perbatch. These have been using pure titanium oxide as a starting materialin their process. Most of the titanium oxide used is produced using pureTiCl₄ produced as per steps indicated in the previous paragraph.

In this invention, a unique production of titanium compounds and metalby sustainable methods using iron-titanium oxide starting material suchas ilmenite, leucoxene, or rutile is described. Sustainable method is aprocess having as low a carbon dioxide emission, as well as having anear zero effluent or waste. Here the iron-titanium oxide compound isprepared by converting the iron portion of the compound to ferrouschloride at low temperatures by using close to stoichiometric amounts ofsulfur and chlorine required for all the iron oxides and the othernon-titanium oxides. The ferrous chloride thus formed is removedrecovering a marketable product of ferrous chloride and the‘sustainable’ titanium oxide starting material containing 93-99% TiO2,which is equivalent to or better than synthetic rutile available. Thiscan be converted to ‘sustainable’ titanium chloride by process shownherein for further conversions to titanium dioxide pigment by presentchloride process or supplied to existing titanium sponge producers,benefitting them in having a ‘sustainable process’.

Alternately, he ‘sustainable process’ titanium oxide starting materialis subjected to metallo-thermic reduction using alkaline earth metals,such as magnesium and or calcium, in preparing the titanium powder byadditional processing by controlled techniques. The purity of titaniumpowder is adjusted by needed secondary processing. The magnesium oxideformed is removed from the titanium powder in a series of steps andrecycled to make the magnesium or calcium metal by electrolysis afterconversion to anhydrous magnesium chloride by unique steps includingsulfo-chlorination.

Prior Art

There is no existing prior art describing as a ‘low carbon dioxide footprint’ titanium oxide, although most of the titanium oxide madeworldwide, using the sulfate process titanium oxide can be considered alow carbon dioxide foot print. But cannot be considered as a sustainableprocess as the co-produced ferrous sulfate or ‘copperas’ is allowed topile up as a waste. Because of that reason, most of the sulfate processin the United States has been closed.

The iron oxide—titanium oxide compounds naturally occur as ilmenite[FeTiO₃], or leucoxene with a higher TiO₂ content, and these areupgraded to synthetic rutile as mentioned earlier. Du Pont corporationpractices a different method where they convert the ilmenite to ironchlorides in an intermediate step and titanium tetra chloride by usingcarbo-chlorination in a selective manner, where the chlorine used forconverting the iron oxide portion of the ore is returned to the processresulting in metallic iron as a co-product, as Glasser H. H. disclosedin patent U.S. Pat. No. 4,017,304. Here the carbo-chlorination iscarried out at temperatures of 950 to 1400° C.—supplying the energyneeded for iron chloride reduction as well as the endothermic heat forthe formation of TiCl₄.

Patents describing production of synthetic rutile precursor by utilizingcarbonaceous reductants and fuels for making the tetrachloride neededfor either the production of the pigment TiO₂ or the titanium sponge byKroll Process, are too numerous to be shown here. Some of theseprocesses are recognized by names as benelite, ishihara, murso, Dupontselective chlorination and Becher process. Muskat. I. E., et al, in1941, described chlorinating iron-titanium oxide containing ore usingcarbon and chlorine or carbonaceous chlorine gases such as phosgene attemperatures in the 600 to 1250° C. range in U.S. Pat. No. 2,245,076.McKinney. R. M, in 1951, discussed carbo-chlorination of ilmenite usingcoke and chlorine in U.S. Pat. No. 2,701,179.

The patents describing the use of sulfur as a reductant as prior art arepresented here, in understanding the novelty of the present invention.Jenness. L. G. showed, in 1931, the use of sulfur higher chlorides andchlorine over the ore to make the metallic chloride vapors of the metalsin the ore, in U.S. Pat. No. 1,834,622. He also teaches use of varyingtemperatures to make the metallic vapors in sequence as a means ofseparating the metals—examples cited include removal of titanium fromaluminum containing ores, and tantalum separation from columbium or tin.

In 1961, Hill. C. T. described suspending ores such as TiO2—rutile, inmolten sulfur [excess sulfur] and chlorinating the ore to make volatilechlorides and sulfur dioxide, in U.S. Pat. No. 2,970,887. The reactionsare carried out using ferric chloride, sulfuryl chloride or chlorine andthe temperatures being in the range 250 to 350° C. while keeping sulfurin the molten liquid state.

Baetz. H. B. et al. discussed converting both iron and titanium inred-mud from alumina production into iron and titanium chlorides usingsulfur chlorides at 350 to 450° C. preventing silicon chlorideformation, using a chlorine to sulfur ratio ˜4.5 to 1, in U.S. Pat. No.3,690,828.

Lumsden J. describes, in U.S. Pat. No. 4,179,489, the chlorination ofilmenite using sulfuryl chloride, chlorine and recycled dimeric ferricchloride vapors in the temperature of 200 to 670° C. range tosimultaneously form ferrous chloride solids [below its melting point]and titanium chloride vapors and sulfur dioxide gas. The processproduces titanium tetra chloride and iron oxide. The iron oxide solidsbeing formed by reaction with oxygen in a second reactor where dimericferric chloride vapors [Fe₂Cl₆] formed is recycled to the first reactor.

The TiCl₄ by any of the different processes is further purified beforebeing used in producing the pigment TiO₂ or the metallic titanium eitheras a powder, or as a spongy agglomerate. The pigment production involvesspecial oxidation to produce submicron nano particles of titaniumdioxides. The production of metallic titanium has been typically carriedout in Kroll Process using magnesium as a reducing agent, or by theHunter or other variations such as Armstrong Process using sodium metalas the reducing agent.

PRESENT INVENTION

The flowchart for Titanium by sustainable Methods is shown in FIG. 1showing various chemical process steps with reactions.

It is to be noted that even naturally occurring rutile has about 5 to 8%iron oxides, and some other oxide impurity. The ilmenite and leucoxeneoccur either as weathered beach sands or as hard rock ores containing 45to 60% TiO₂, 35 to 55% iron oxides and other oxides such as vanadium andother metals. The industrial waste product from bauxite conversion toalumina typically contains recoverable titanium oxides mixed with ironoxides which may also be recovered by the present sustainablechlorination techniques. The present invention uses sulfur as areductant along with chlorine in producing a ‘low carbon dioxidefootprint process’ for making ‘sustainable’ titanium compounds andmetal. The invention discloses methods of selectively removing the ironas saleable ferrous chloride solution, and producing near pure‘sustainable’TiO₂ produced at a low cost along with cleaned sulfurdioxide for conversion to marketable sulfuric acid. The inventionfurther discloses methods for making low cost, ‘sustainable TiCl₄’ inone of the embodiments and a ‘sustainable Ti powder’ in anotherembodiment.

The present invention teaches to carryout the sulfochlorination of ironbearing titanium oxide minerals in sequential steps. The first step willuse chlorine in stoichiometric proportion to convert all the ironcomponent of ilmenite alone without converting the titanium component ina Stage One Reactor. Ilmenite mineral is a compound of iron oxides inthe divalent and trivalent states along with titanium dioxide. Itusually depicted as FeTiO3 or as FeO.TiO2, containing some Fe2TiO5 orFe2O3.TiO2. The invention teaches the use of sulfur as an oxygen removerduring the chlorination to avoid carbon oxide release.

The reactions taking place in the first step will be

FeTiO3+0.5S+Cl2=FeCl2+0.5 SO2+TiO2   [Reaction 1]

0.5 Fe2TiO5+0.75S+1.5 Cl2=FeCl3+0.5TiO2+0.75SO2   [Reaction 2]

The sulfur dioxide coming in the off-gas will be recovered into sulfuricacid using conventional catalytic reactors. The temperature of thereactions will be controlled to minimize the formation of ferricchloride vapors—and the equipment in the offgas will have conventionalstepwise condensers to remove condensable vapors such as fern chloride,etc to send the cleaned sulfur dioxide vapors for recovery as sulfuricacid. The ferrous chloride, FeCl2 and the ferric chloride, FeCl3 arewater soluble matter in the process. The solid reaction products ofcombined Reactions 1 and 2 will be washed with water to filter outessentially clean titanium dioxide filter cake, TiO2, and remove theFerrous chloride containing some ferric chloride solution as amarketable product. The water washed filter cake titanium dioxide maycontain some other insoluble impurities and these are removed in Stage 2Purification of TiO2; using methods such as flotation, followed bysecondary sulfo-chlorination to remove rest of the iron while loosing aminimal quantity of titanium as titanium tetrachloride to vapors whichis recoverable from the condensers before releasing pure sulfur dioxideto its recovery process.

Most often the first step alone does not remove all the iron from theTiO2 as was described earlier. In order to assure cleanliness of theproduct, the filter cake is dried and put through an intermediatesecondary sulfo-chlorination step. The amount of chlorine used in thisstep will be that required to remove iron and other chlorinatableimpurities with minimal loss of titanium to result in essentially pureTiO2 applicable to pigment production. In this step some TiO2 will belost by Reaction C to the off-gas as TiCl4 which can be recovered inconventional stepwise condensers which remove ferric chloride and othervolatile chlorides coming with sulfur dioxide, before the sulfur dioxideis sent to be recovered as sulfuric acid.

TiO2+S+2Cl2=TiCl4 [vapor]+SO2   [Reaction 3].

This invention is a sustainable process as the oxygen remover sulfur isnot wasted and is recovered from the off-gas as marketable sulfuric acidand thus minimizes the cost of production of titanium dioxide.

The current invention which produces high purity TiO2 can not only besold as pigment grade material, but also be used for direct reduction totitanium metal, using magnesium or calcium reducing agents in ametallothermic process. Several examples are shown below as how theinvention is put into practice.

The high purity TiO2 can be converted to TiCl4 for use in existingprocesses, or marketed. The metallo thermic reduction process of TiO2 isexothermic, and the present invention teaches rgy to recover this asCogenerated ento improve the sustainability of the process further. Thetitanium metal produced in this step produces a product containing themetal [magnesium or calcium] oxide along with titanium powder; thismixture is separated sending the metal oxide for recycling into metallicreductant. This recycle product will require energy which will besupplied by alternate energy such as solar or wind energy hybrid withgrid energy or cogenerated energy from earlier step.

EXAMPLE 1

The ore body is analyzed for its iron, titanium and other elementaloxide content; and the ore particle size is reduced to be in the lessthan 200 mesh or preferably less than 325 mesh size. The first stagepartial chlorination is carried out in the 70 to 250° C., preferably inthe 125 to 150° C. range with the following mole proportions of 0.5moles of sulfur per mole of contained iron, and one mole of chlorine permole of contained iron.

FeO.TiO₂+0.5S+Cl₂==FeCl_(2[s])+TiO_(2[s])+SO_(2[g])  [A]

The sulfur can be mixed with the ore before being fed to the reactor.The reaction can be carried out in a rotating tubular reactor or in amoving or fluid bed reactor while controlling the reaction temperature,to form solid ferrous chloride mixed with the ore body. The proportionsof reagents are made to avoid wasted formation of ferric chloride vaporsor titanium chloride vapors in this stage. The vapor formed is mainlysulfur dioxide, along with possible minor quantities of low temperaturevolatile chlorides and water vapor from any moisture in the ore. Thesulfur dioxide vapor is sent to the acid production step for recovery,following necessary intermediate scrubbing.

The solids from stage 1 reactor is then taken to a water washing stepwhere controlled amount of water is added to make 30 to 35% ferrouschloride which is saleable to the water treatment chemical market intreating sewage and the like matter. Then the undissolved solid presentis upgraded TiO₂ with a 97 to 99% content. It is to be understood thatthis procedure is applicable to upgrading naturally occurring rutile oreven the presently marketed synthetic rutile. The solids are thenfiltered with the filter cake washing to remove residual ferrouschloride, and taken to a secondary step of drying associated withfurther upgrading the TiO2 to the 99% or better grade. This step iscarried out by adding small quantities of sulfur and twice the [sulfur]molar amount as chlorine and adjusting the reactor temperature to 500 to900° C., preferably about 700° C. removing the rest of the iron, andother volatile chlorides along with less than about 2 to 5% of theTiCl₄—the amount being controlled by keeping the temperature of theprocess and additives managed to keep a balance between the exothermicheats of iron chlorination, sulfur oxide formation and the endothermicheat for TiCl₄ formation. The gases formed in this step include Hydrogenchloride, sulfur dioxide, iron chloride vapors and titanium chloridevapors. These gases are processed to recover the sulfur dioxide to thesulfuric acid plant, and the iron chloride to the marketable ferrouschloride solution, and recovered Titanium tetrachloride liquid forfurther use.

FeO+0.5S+1.5Cl₂+TiO₂==xFeCl_(3[v])+((1−x)2)Fe₂Cl_(6[v])+SO_(2[g])+TiO_(2[pure]—)  [B]

The second stage pure titanium dioxide material can then be processed inone of several ways.

EXAMPLE 2

In one embodiment of the present invention, it is converted to puretitanium tetrachloride in a third stage sulfo-chlorination reactor usingone mole of sulfur per mole of titanium dioxide, and two moles ofchlorine per mole of upgraded titanium dioxide. The titaniumtetrachloride from the third reactor would have its own off-gas systemto recover condensed TiCl₄ and send the sulfur dioxide to sulfuric acidrecovery plant. This makes a low carbon dioxide foot print and ‘asustainable’ TiCl₄

TiO₂+S+2Cl₂==TiCl_(4[v])+SO_(2 [g])  [C]

TiCl_(4[v])[condensing scrubber→]TiCl_(4[1])  [D]

SO_(2[g])→Sulfuric acid  [E]

Reaction of chlorination TiO₂ is endothermic, added heat if necessary issupplied by co-burning sulfur with oxygen which produces sulfur dioxidewhich is another product handled by the process.

EXAMPLE 3

In another embodiment of the present invention, the second stage puretitanium dioxide is converted to titanium metal by reaction withmagnesium metal in a specially designed reactor where the temperature iscontrolled—in producing initially a mixture of titanium powder andmagnesium oxide. The temperature is controlled in the 400 to 900° C.range and preferably around 500 to 600° C.

The mixture of titanium powder and magnesium oxide is first processed byslurrying in water, followed by flotation of magnesium hydroxide to afroth stream in flotation equipment such as a flotation column, whileleaving the titanium powder to the heavier tail stream. The heavier tailstream titanium powder is analyzed for its purity from inclusions, andunremoved magnesium and titanium oxides. Following such analysissufficient amount of a suitable acid is added avoiding dissolution oftitanium powder. Then the powder is filtered, filter cake dried withheated gas inert to the powder—thus making the precursor for near netshape parts by powder metallurgy or converted into an ingot for furtherprocessing the metal in a ‘sustainable’ manner and at a lower energy andcost.

EXAMPLE 4

The process shown in example 3 is carried out using calcium instead ofmagnesium providing similar products.

EXAMPLE 5

The magnesium hydroxide froth from flotation step is filtered to recoverthe magnesium values as a filter cake. The filter cake is converted tomagnesium sulfate monohydrate by reaction with concentrated sulfuricacid. The magnesium sulfate monohydrate undergoes dehydration around250° C. in a fluid bed drier, preferably operated with a solar thermalheat exchange mechanism. The anhydrous magnesium sulfate is thentransferred to a second fluid bed reactor where it is reacted withsulfur and chlorine in the 200 to 600° C., preferably around 350° C.making anhydrous magnesium chloride solids as a feed to magnesiumchloride electrolysis unit. The sulfur dioxide formed is recycled to thesulfuric acid recovery process. The molten magnesium recovered from theelectrolysis is processed to make the magnesium powder recycled to thetitanium powder plant, while the chlorine is recycled to making theanhydrous magnesium chloride in this sustainable production technique.

MgO+H₂O==Mg(OH)₂   [F]

Mg(OH)₂+H₂SO₄==MgSO₄.H₂O+H₂O  [G]

MgSO₄.H₂O==MgSO₄+H₂O  [H]

MgSO₄+S+Cl₂==MgCl₂+2SO₂  [I]

MgCl2==Mg+Cl2[electrolysis]

EXAMPLE 6

Exact similar steps as in example 5 is carried out when calcium used asa reductant in the titanium powder process recovering the calciumhydroxide filter cake from the froth from flotation step in convertingit back to calcium metal for the reduction of pure TiO₂

EXAMPLE 7

The magnesium hydroxide froth from flotation step is filtered to recoverthe magnesium values as a filter cake. The filter cake is converted tomagnesium sulfite by reaction with sulfite. The magnesium sulfite isthen dried in a rotary or fluid bed drier, preferably operated with asolar thermal heat exchange mechanism. The anhydrous magnesium sulfiteis then transferred to a second rotary or fluid bed reactor where it isreacted with sulfur and chlorine in the 200 to 600° C., preferablyaround 350° C. making anhydrous magnesium chloride solids as a feed tomagnesium chloride electrolysis unit. The sulfur dioxide formed isrecycled to the sulfuric acid recovery process. The molten magnesiumrecovered from the electrolysis is processed to make the magnesiumpowder recycled to the titanium powder plant, while the chlorine isrecycled to making the anhydrous magnesium chloride in this sustainableproduction technique.

Mg(OH)₂+SO₂==MgSO₃+H₂O  [J]

MgSO₃+0.5S+Cl₂==MgCl₂+1.5SO₂  [K]

EXAMPLE 8

Exact similar steps as in example 7 is carried out when calcium used asa reductant in the titanium powder process recovering the calciumhydroxide filter cake from the froth from flotation step in convertingit back to calcium metal for the reduction of pure TiO₂.

We claim:
 1. A sequential process for the production of titaniumcompound and metal by sustainable methods, the said process is carriedout utilizing iron oxide containing ores of titanium such as ilmenite,leucoxene, hard rock ilmenite, rutile as well as man-made intermediatecompounds such as synthetic rutile along with the use of sulfur andchlorine in a controlled fashion of step-wise addition of the reagentand control of temperatures.
 2. The said process per claim 1, startswith the use of stage 1 reactor carrying out controlled chlorination ofiron in the mixed oxide to ferrous chloride solids along with unreactedtitanium solids, and conversion of the said ferrous chloride tomarketable ferrous chloride solution in water of suitable concentration,and making a high grade synthetic rutile with 97-99% TiO₂ which can beeither marketed or taken to a next step. The controlled chlorination instage 1 reactor is carried out by adding stoichiometric amount of sulfurand chlorine needed for conversion of iron to ferrous chloride solids ata temperature of 70 to 250° C., preferably around 130 to 150° C. Theproducts being marketable ferrous chloride solution, high gradesynthetic rutile and sulfur dioxide which is sent to a treatment plantmaking compressed SO₂, or converted into saleable sulfuric acid.
 3. Thesaid process per claim 1, continues with a stage 2 reactor which usesthe high grade synthetic rutile of stage 1 [or purchased rutile ofequivalent high titanium dioxide content] in a purification stage 2 byremoval of rest of the iron as ferric chloride and dimeric ferricchloride vapors along with minimal amount of titanium chloride vaporsassuring iron removal. The controlled chlorination carried out by addingwith slightly excess of stoichiometric amount of sulfur and chlorineneeded for conversion of iron to ferrous chloride solids at atemperature of 500 to 900° C., preferably about 700° C. The productgases are processed to recover the sulfur dioxide to the sulfuric acidplant, and the iron chloride to the marketable ferrous chloridesolution, and recovered titanium tetrachloride liquid for further use.The solid product is the ‘sustainable pure titanium dioxide’.
 4. Thesaid process per claim 1, using the stage 2 ‘sustainable pure titaniumdioxide’ into a marketable milled pigment of different brightness thanconventional pigments.
 5. The said process per claim 1, continues tostage 3 reactor for producing ‘sustainable titanium tetrachloride’ usingthe stage 2 ‘sustainable pure titanium dioxide’ with sulfur and chlorineand carrying out the reaction in a controlled fashion, [with added heatif necessary which is supplied by co-burning sulfur with oxygen] alongwith sulfur dioxide. Parts of the added heat may be supplied by usingrenewable energy, thus improving the sustainability.
 6. The said processper claim 1 continues forusing the ‘sustainable pure titanium dioxide’into ‘sustainable titanium metal’ in a metal producing step alternate 1,by reaction with magnesium metal by controlling the temperature ofreaction in the 200 to 900° C. range and preferably around 500 to 600°C. The co-formed magnesium oxide and titanium powder is initiallyprocessed by flotation recovering the magnesium oxide values as amarketable product or for reprocessing into magnesium, and minimizingthe acid needed to convert the titanium powder into a pure material forfurther processing into near net-shape final product and or making aningot of pure titanium for further applications.
 7. The said process perclaim 1, continues for using the ‘sustainable pure titanium dioxide’into ‘sustainable titanium metal’ in a metal producing step alternate 2,by reaction with calcium metal by controlling the temperature ofreaction in the 200 to 1300° C. range and preferably around 500 to 1050°C. The co-formed calcium oxide and titanium powder is initiallyprocessed by flotation recovering the calcium oxide values as amarketable product or for reprocessing into calcium, and minimizing theacid needed to convert the titanium powder into a pure material forfurther processing into near net-shape final product and or making aningot of pure titanium for further applications.
 8. The said process perclaim 1, continues for using the ‘sustainable pure titanium dioxide’into ‘sustainable titanium metal’ in a metal producing step alternate 3,by reaction with calcium-magnesium alloy by controlling the temperatureof reaction in the 200 to 1300° C. range and preferably around 500 to1050° C. The co-formed calcium-magnesium oxide and titanium powder isinitially processed by flotation recovering the calcium magnesium oxidevalues as a marketable product or for reprocessing into calciummagnesium alloy, and minimizing the acid needed to convert the titaniumpowder into a pure material for further processing into near net-shapefinal product and or making an ingot of pure titanium for furtherapplications.
 9. The said process per claim 1 in which the sustainabletitanium metal formed, by any of the three alternate metal produingsteps using an alkaline earth metal, such as magnesium or calcium, thealkaline earth oxide recovered from the flotation froth as an alkalineearth hydroxide filter cake is converted back to alkaline-earth metalneeded for reduction. This is done in a series of steps—by conversion ofthe alkaline earth hydroxide to an alkaline earth sulfate hydrate orsulfite followed by being dried into an anhydrous alkaline earth sulfateor sulfite. The anhydrous alkaline earth sulfate or sulfite is treatedwith sulfur and chlorine producing an anhydrous alkaline earthchloride—such as magnesium or calcium chloride—preferably in a solidstate—by controlling the reaction temperature, and minimizing the energyneeded. The anhydrous alkaline earth chloride is then used as a feedmaterial to an electrolytic cell where the metal and chlorine arerecovered for recycle into the process step. The sulfur dioxide formedin the sulfo-chlorination step is recycled or recovered through thesulfuric acid process.
 10. Conversion of alkaline earth oxide orhydroxide [MgO or CaO] to sulfites using SO₂, followed by drying to makeit anhydrous sulfite, then subjecting the anhydrous sulfite tosulfo-chlorination making anhydrous alkaline earth chloride—suitable forelectrolysis to produce alkaline earth metal and chlorine. The sulfurdioxide formed in the preparation of anhydrous alkaline earth chlorideis recycled.
 11. Conversion of alkaline earth oxide [MgO or CaO] orhydroxides to sulfates using H₂SO₄, and crystallizing to preferablylower hydrates of sulfate [such as kieserite or gypsum]. The lowerhydrates of alkaline earth sulfates thus formed or naturally occurringalkaline earth sulfate hydrates are treated by mild calcination to makeanhydrous sulfate, then subjecting the anhydrous sulfate tosulfo-chlorination making anhydrous alkaline earth chloride—suitable forelectrolysis to produce alkaline earth metal and chlorine. The sulfurdioxide formed in the preparation of anhydrous alkaline earth chlorideis recycled.